K201 (JTV519) is a Ca2+-Dependent Blocker of SERCA and a Partial Agonist of Ryanodine Receptors in Striated Muscle

K201 (JTV-519) may prevent abnormal CA(2+) leak from the sarcoplasmic reticulum (SR) in the ischemic heart and skeletal muscle (SkM) by stabilizing the ryanodine receptors (RyRs; RyR1 and RyR2, respectively). We tested direct modulation of the SR CA(2+)-stimulated ATPase (SERCA) and RyRs by K201. In isolated cardiac and SkM SR microsomes, K201 slowed the rate of SR CA(2+) loading, suggesting potential SERCA block and/or RyR agonism. K201 displayed CA(2+)-dependent inhibition of SERCA-dependent ATPase activity, which was measured in microsomes incubated with 200, 2, and 0.25 µM CA(2+) and with the half-maximal K201 inhibitory doses (IC50) estimated at 130, 19, and 9 µM (cardiac muscle) and 104, 13, and 5 µM (SkM SR). K201 (≥5 µM) increased RyR1-mediated CA(2+) release from SkM microsomes. Maximal K201 doses at 80 µM produced ∼37% of the increase in SkM SR CA(2+) release observed with the RyR agonist caffeine. K201 (≥5 µM) increased the open probability (Po) of very active ("high-activity") RyR1 of SkM reconstituted into bilayers, but it had no effect on "low-activity" channels. Likewise, K201 activated cardiac RyR2 under systolic CA(2+) conditions (∼5 µM; channels at Po ∼0.3) but not under diastolic CA(2+) conditions (∼100 nM; Po < 0.01). Thus, K201-induced the inhibition of SR CA(2+) leak found in cell-system studies may relate to potentially potent SERCA block under resting CA(2+) conditions. SERCA block likely produces mild SR depletion in normal conditions but could prevent SR CA(2+) overload under pathologic conditions, thus precluding abnormal RyR-mediated CA(2+) release.